The present invention relates to a method for producing a graytone mask having a graytone part to obtain light transmission in an intermediate amount of between an opaque part and light-transmission part.
In recent years, attempts have been made to reduce the number of mask sheets by using graytone masks in the process for making TFT (Thin Film Transistor) in the field of large-sized LCD masks (as set forth in the monthly FPD Intelligence, May, 1999).
As shown in FIG. 8A, such a graytone mask has an opaque part 1, a transmission part 2 and a graytone part 3. The graytone part 3 corresponds to an area in which there is formed an opaque pattern 3a of below or equal to the resolution limit of an exposure apparatus for a large-sized LCD using the graytone mask and is designed to selectively change the thickness of a photoresist film by decreasing the light transmitted through this area so as to decrease the amount of irradiation due to the area. Normally, the opaque part 1 and the opaque pattern 3a are formed with films that are made of the same material such as chromium (Cr) or a chromium compound and have the same thickness.
The resolution limit of the exposure apparatus for the large-sized LCD using the graytone mask is about 3 μm in the case of an exposure apparatus of a stepper type and about 4 μm in the case of an exposure apparatus of a mirror projection type. Consequently, the space width of a transmission part 3b in the graytone part of FIG. 8A is set at less than 3 μm and the line width of the opaque pattern 3a of below or equal to the resolution-limit of the exposure apparatus is set at less than 3 μm, for example. When the exposure apparatus for the large-sized LCD is used for light exposure, as the exposure light transmitted through the graytone part 3 as a whole is deficient in the amount of light exposure, positive photoresist are left on a substrate though the thickness of the positive photoresists exposed to light via the graytone part 3 solely decreases. This phenomenon is called as graytone effect. More specifically, there arises a difference in solubility of resists in developing liquid between parts corresponding to the ordinary opaque part 1 and to the graytone part because of difference in the amount of light exposure and this results in, as shown in FIG. 8B, making a part 11 corresponding to the ordinary opaque part 1 as thick as about 1.3 μm, making a part 13 corresponding to the graytone part 3 as thick as about 0.3 μm and making a part corresponding to the transmission part 2 a part 12 without resists, for example. A first etching of a substrate as a workpiece is carried out in the part 12 without the resists. Then, the resists in the thin part 13 corresponding to the graytone part 3 are removed by ashing and the like and a second etching of this part is carried out. The etching processes are performed with one mask instead of two masks as conventionally used in order to reduce the number of masks for use.
In producing a graytone mask of the above type, an opaque film is formed of chromium, a chromium compound or the like over a glass substrate. A resist film is formed thereon, most commonly, by a spin coat technique, and then patterning is done with the pattern data including graytone pattern data to develop the resist film, thereby forming a resist pattern. This resist pattern is used as a mask to etch the opaque film, followed by stripping away the resist pattern.
In the graytone mask, because the amount of light transmission through the graytone part is controlled by the dimensions of a graytone pattern, dimensional accuracy is required strictly on the graytone pattern. Namely, in case there are variations in linewidth dimensions in the graytone pattern on a substrate surface, variations of transmissivity occur in the graytone region. It can be considered, as one method for preventing such variations in plane pattern dimensions, to use a resist film having less variations in film thickness in order for forming a graytone pattern. However, in applying resist by the spin coat technique most commonly employed in the conventional, the resist film inevitably rises at the substrate peripheral edge by rotation. There has been a problem that there is limitation in reducing the on-plane variations of resist-film thickness due to the effect of rise at the peripheral edge or the like. Particularly, in producing a graytone mask for a display device necessarily using a large-sized substrate of 330 mm or greater in one side, there is difficulty in producing a resist mask evenly throughout the large area thereof.